This invention relates to a programmable controller, and more particularly to a programmable controller and related method of controlling execution of a sequence program.
FIG. 1 is a diagram showing a conventional programmable controller. A sequence execution control part 11 is connected by a common address bus 12 and a data bus 13 to a program memory 14 and a data memory 15. Program memory 14 stores a group of sequence instructions consisting of an instruction part 14a and an operand part 14b. Data memory 15 stores computation data such as process input/output data and internal data. Sequence execution control part 11 executes sequence computations based on computation data from data memory 15 and an instruction fetched from program memory 14.
That is, sequence execution control part 11 inputs sequence instructions into an instruction register 16 through a data bus 13, from the program address in program memory 14 designated through common address bus 12 by an instruction pointer 17. Next, through common address bus 12, instruction register 16 designates, in data memory 15, operand part 14b, i.e. the address on data memory 15, of this sequence instruction and inputs it, through data bus 13, into data register 18. Finally, based on the content of instruction register 16 and the content of data register 18, a binary logic computation, which constitutes the main part of the sequence instructions, is executed by a bit computation processing part 19.
FIG. 2 shows a conceptual view of the operation of the programmable controller in this case. As shown in FIG. 1, the execution time for one instruction of the binary logic instructions constituting the sequence instructions is the sum of the instruction fetch time, the operand data input time, and the instruction execution time. Furthermore, the time required for memory access for instruction fetch or operand data input is longer than the time required for the binary logic computation that represents the instruction execution time.
The above points up to the problem that, in the conventional programmable controller, memory access time represents a large proportion of the sequence instruction execution time, which otherwise mainly consists of binary logic computation. This memory access time therefore results in loss time, preventing the execution of sequence instructions from being carried out at high speed.